The telescope aperture At(k) [cm2].
At(k) is a function of the scan mirror position (k),
because as the scan mirror moves, the two NIS gratings are `seeing'
different parts of the NIS telescope aperture.
The variations, around a median value of 27-28 cm2 (for NIS 1-2)
are considerable ( ~ 20%) over the full extent of the scan mirror positions
(4¢).
The CDS routine GET_EFF_AREA provides the
pre-flight values of At(k), and these
have been confirmed by in-flight measurements
(W. Thompson, priv. comm.).
Figure 25: [Figure from Bill Thompson]
The heavy solid line shows the theoretical curve based on ray
tracing.
Note that the scan mirror position goes from 68 to 188, with
128 representing the centered
"home" position. Since the scan mirror moves from right to left, this
plot is reversed from the normal raster scan pattern. The narrow line
shows the measured
behavior of the instrument from the SYNOP_F observations for the Mg IX
368 Å line. This curve is normalized so that a quadratic fit through
the data will pass
through 26.9 square cm at the home mirror position of 128. Once can
see that the agreement between the Mg IX measurements and theory is
quite good. Also
plotted as a dotted line is the behavior derived from the Fe XVI 361 A
line. These data are much noisier, but still follow the same general
trend.
Figure 26: [Figure from Bill Thompson]
The heavy line shows the theoretical value. The narrow solid line
shows the behavior derived from He I 584 A, and the dashed line shows
the behavior
derived from O V 630 A, both normalized to the theoretical value of
28.0 square cm at a mirror position of 128. There's good agreement
between He I and O V.
Both follow the general trend predicted by theory, although the data
do not show as much variation as theory predicts.
